Body dysmorphic disorder (BDD) is a severe, often chronic, and common disorder consisting of distressing or impairing preoccupation with imagined or slight defects in appearance. Individuals with BDD have very poor functioning and high rates of hospitalization and suicidality. The rate of completed suicide, while preliminary, appears markedly high. A majority of patients receive surgical, dermatologic, or dental treatment for BDD, which is usually ineffective. Our prior work has shown that SRIs are often efficacious for BDD, but patients may not agree to try medication or may not benefit from it. Thus, there is a critical need for an efficacious psychosocial intervention for this severe disorder. Because BDD differs in important ways from other disorders, psychotherapies for other disorders are not adequate for BDD. There is no adequately tested psychosocial treatment of any type for BDD. An evidence-based treatment for this unique disorder is greatly needed. Cognitive-behavioral therapy (CBT) is the only psychosocial treatment for BDD that is fully developed and now ready for formal testing. Data from case series and studies using waitlist controls suggest that BDD- specific CBT is very promising for BDD. With an R34 grant, we developed a personalized, modular, manualized CBT treatment (CBT-BDD) that specifically targets BDD's unique and complex symptoms. The R34 study, which used a waitlist control group, suggests CBT-BDD is acceptable to patients, feasible to implement, and appears efficacious for BDD, associated symptoms, and functional disability. However, no study has adequately tested CBT's efficacy for BDD -- for example, by comparing it to another treatment that controls for therapist time, attention, and other non-specific treatment elements. Thus, there is a pressing clinical and public health need to test the most promising psychosocial treatment for BDD. This Collaborative R01 application's primary aim is to conduct the first study to adequately examine the efficacy of CBT for BDD. This study will compare our personalized and manualized CBT-BDD to manualized supportive psychotherapy (SPT). SPT appears to be the psychosocial treatment most often used for BDD, yet it appears to have a low response rate. Massachusetts General Hospital/Harvard Medical School and Rhode Island Hospital/Brown University will randomize 120 adults to CBT-BDD or SPT over 24 weeks followed by 3- and 6-month follow-up assessments. The primary outcome will be change in BDD severity assessed by a blinded Independent Evaluator. Secondary aims will examine change in depressive symptoms, delusionality of BDD beliefs, psychosocial functioning, and quality of life, as well as predictors of improvement in BDD. The United Kingdom's National Health Service treatment practice guideline on BDD and a Cochrane Collaboration review on BDD underscore the dearth of treatment research and call for more intervention research on BDD. The proposed study will fill a major gap in knowledge and patient care by conducting the first adequate test of any psychosocial treatment for a unique, disabling, common, and understudied disorder.